In connecting two pipes together, it is often advantageous to counterbore one pipe end so that certain dimensions of the end, for example wall thickness and/or inside diameter, can be appropriately matched with the end of the other pipe to facilitate proper welding of the two pipe ends together. Precision and accuracy are desired when counterboring pipe ends. The machining device must be held firmly in relation to the pipe in order to achieve these results. Further, in many instances a pipe in the field is out of round due to rough handling or manufacturing limitations. This is especially common in pipes having large diameters or having relatively thin walls compared to the diameter, such as those used for oil and gas pipelines. It is therefore desirable that the machining tool follow the actual circumferential wall surface of the pipe with high precision to ensure uniform machining of the counterbore.
U.S. Pat. No. 5,171,110 to Pettifer et al. (the Pettifer '110 patent) and U.S. Pat. No. 5,361,659 to Hanson (the Hanson '659 patent) disclose two machining devices that rotate a cutting tool against the inside circumferential surface of the pipe end and advance it axially into the pipe until a counterbore has been formed. In the Pettifer '110 patent, the cutting tool is pivotally connected to a rotating flywheel and the cutting tool is pressed radially outward against the inside surface of the pipe by being rotated about its pivot point. In the Hanson '659 patent, the cutting tool is slidably mounted on a pair of spaced-apart parallel guide structures connected to a rotating flywheel and the cutting tool is pressed radially outward against the inside surface of the pipe by being slid radially along the guide structures.
In both the Pettifer '110 patent and the Hanson '659 patent, the radial depth of cut of the counterbore is controlled by a guide wheel which is connected to the cutting tool and pressed against the inside surface of the pipe at a location offset axially ahead of the position of the cutting tool. The guide wheel follows the inside contour of the pipe causing the cutting tool to oscillate radially so as to reproduce the inside contour of the pipe in the counterbore section.
It has been found that counterbore devices using a guide wheel which presses against the inside surface of the pipe can produce defects in the counterbore where the pipe interior has surface defects, for example weld seams, weld spatter, or pipe scale, or where the pipe interior is contaminated with dirt or other debris. Such surface defects or contaminating materials are not uncommon in the pipes used, for example, on cross-country oil and gas pipelines. The location of these defects or contaminants within the interior of the pipe makes their detection problematic, and even if detected, they can be hard to remove. A need therefore exists, for an apparatus for counterboring a pipe which does not control the radial depth of cut of the cutting tool by pressing a guide member against the inside surface of the pipe.
It has also been found that counterbore devices using a guide wheel which is offset axially (i.e., parallel to the longitudinal axis of the pipe) from the position of the cutting tool can produce defects in the counterbore where the circumferential profile of the pipe changes in the axial direction. For example, if only a few inches of the end portion of the pipe are out of round due to field handling, while the remaining portion of the pipe is in round, then an apparatus having a guide wheel which is offset axially from the cutting tool can produce a counterbore which does not accurately match the circumferential profile of the counterbored section of pipe. A need therefore exists, for a counterbore device which uses a guide wheel which is not offset axially from the location of the cutting tool.
For reasons of safety, expense, and convenience, ultrasonic inspection means are rapidly replacing radiographic (x-ray) inspection means for the testing of field welds made during the construction of pipelines. To facilitate such ultrasonic inspection, many pipes are now furnished with a precision ground outer surface at each end. These ground areas at the ends of the pipe typically have a very smooth finish, are free of surface defects or contamination, and are easily accessible for the removal of any defects or contamination which may be present. A need therefore exists, for a counterbore method and apparatus which utilizes the outer surface of a pipe end for controlling depth of cut of a counterbore.